For me, this stuff is gold.
Excellent! I'm glad to help!
Jesus... just typing this I'm all, "I want to a build a subsector just for this scenario!" with a hunt across the worlds to put down the mother▮▮▮▮▮▮ers who did this. Like, right now.
This is some great stuff for story, setting, and RPG adventures.
That was essentially what happened in that campaign, a related campaign had essentially the Carbon Plague from CP2020 start, with the party playing the part of the Imperial "Wildfire" protocol that also locked things down in an effort to contain and understand it...
For me, as I've been thinking this through (again, it's been a few years since I ran that game) I think the key part is a matter of scale. Perhaps it is understanding that on an interstellar scale, nukes are certainly bad, but that they are almost the equivalent of a really big car bomb. With Nuclear Dampers, anti-radiation meds, etc while it certainly is a "bad thing" it isn't quite the threat that it is now.
So the Rockdrop scenario almost falls into two categories - the Nightmare, "Ellie Scenario" (from E.L.E. "Extinction Level Event") that destroys a world, and the "Crater City Scenario" where someone drops a rock and wipes a city off the map.
As we've noted in the discussion, getting the size and velocity needed for the "Ellie Scenario" is harder rather than easier. I expect (and recall from way back when I computed things) that what's needed for "Crater City" is a heck of a lot less.
(Unless, of course, you get ahold of whatever Black Project superweapon that the Imperium has stashed in one of their Megiddo Depots in deep space - then all bets are off. You're talking Osmium Bolt Projectiles mounted inside small craft shells with multistage high-thrust systems, reactive and ablative armor, along with weasel missile drones to draw off fire and project a false target trail...)
Amusingly, I don't actually think that even these "Nightmare Scenarios" are what keep the Intelligence up at night like their equivalents do ours. What has them utterly terrified is somebody finding an Ancient weapon that they have no idea of how to counter. Not even a planet-destroying superweapon, even just some kind of goofy tech that accidently gets used as a weapon, or an actual weapon that ignores everything - Relativity Guns, Stellar Igniters, Singularity Generators, etc.
That's the real mission of the Ancient's Foundation IMTU and why it's under the sponsorship of the Imperial Family - investigating and searching for the next Black Globe cache.
D.